It is known that alumina the industrial aluminum oxide (Al.sub.2 O.sub.3), is obtained by heating at a high temperature (800.degree.-1400.degree. C.) the alumina trihydrate (Al.sub.2 O.sub.3.3H.sub.2 O) produced in the Bayer process. By careful control and an accurate supervision of the manufacturing conditions, it is possible to obtain different varieties or types of alumina, depending on the calcination degree, chemical purity, particle size distribution and crystallographic characteristic thereof.
Depending on the calcination degree of the hydrate, the obtained products vary in water content and in crystallographic structure, resulting in different types of alumina according to the contents in alpha (.alpha..Al.sub.2 O.sub.3 or corindon) or gamma (.gamma..Al.sub.2 O.sub.3) forms or in other special transition states.
Special aluminas of high calcination degree are those which have a high content in alpha phase and they have uses other than the manufacture of aluminum.
It is known that metallurgical alumina is used for producing aluminum by electrolytic reduction.
The alumina trihydrate, due to its considerable water content, can not be used directly in electrolytic reduction tanks and must be submitted to a calcination stage before being used as anhydrous aluminum oxide or alumina (Al.sub.2 O.sub.3) in the electrolytic tanks for aluminum production.
The Bayer process, as it is well known, involves the digestion of bauxite in a caustic medium under high pressure and at high temperature. The solved alumina is recovered from the resulting liquor by crystallizing it and precipitating it as trihydrate, which, after some steps such as classification, filtration and washing, is submitted to a calcination process in order to eliminate its water content before being used as metallurgical alumina for aluminum manufacture.
When this hydrate is calcined in conventional calcination installations, such as rotary or fluid-bed furnaces, a considerable amount of powder is produced due to the thermic and/or mechanical shock over the particles. These thin generated products must be retained inside the calcination system in order to prevent from atmospheric contamination and from the subsequent product losses. This is generally carried out by powder collecting mechanisms, such as electrostatic precipitators and the like, and the partially calcined alumina powder is commonly known as electrofiltre powder.
This recovered alumina powder is generally characterized by a particle size pattern, there being a higher rate of those having a size below 44 microns. Most frequently, the 90% by weight of alumina powder has a particle size below 44 microns. Additionally, the powder consists of mixture of calcined, partially calcined and non-calcined particles and, consequently, the water content thereof, determined by the fire loss assay (LOI) can vary between wide limits, for instance, between about 1 and 35% by weight. These properties of the alumina powder make it an inadequate by-product for the manufacture of aluminum and its removal creates serious problems, since its rate may reach sometimes a 5-10% with respect to the total amount of alumina yielded in the calcination. Thus, in case of an alumina calcination installation with a production capacity of 500,000 tons each year, the losses by alumina powder could reach the amount of 25-50,000 tons a year. So as to reduce this important loss of the by-product in certain calcination installations, a part of the generated alumina powder is mixed with the produced calcined alumina. The mixing, however, cannot eliminate the problem, since the calcined alumina always contains particles smaller than 44 microns and this content can not exceed certain acceptable limits, established as alumina specification by the operators of the reduction installations.
Due to this, only a relatively small part of the powder can be used, besides the problem of the particle size, the water content (LOI) of the powder also affects negatively the quality of the calcined alumina. Another method for reducing the gathered quantity of alumina powder consists of recycling the powder to the digestion stage, whereat it is resolved in the caustic medium so as to yield alumina trihydrate, which is recovered by crystallization and precipitation. This method, although it solves the problem of the effect of the powder on the quality of the produced alumina, is not economically satisfactory, since the powder redissolution implies the reprocessing thereof, the whole productivity of the plant being thus reduced in a direct relationship with respect to the amount of resolved alumina powder.
Other processes have been suggested for using the powder, either processed or directly, as seeding at the precipitation stage; for example of the U.S. Pat. Nos. 4,051,222 (Gnyra Sept. 27, 1977), 4,568,527 (Anjier and others, Feb. 4, 1986), etc. These alternatives may create some control problems for the precipitation process, increase the plant costs as the products are recycled through precipitation and calcination and, in any case they imply the introduction of alumina in the alumina hydrate crystallization circuit, thereby contaminating the produced hydrate. In plants of alumina production by the Bayer process, a part of the manufactured trihydrate, instead of being calcined in order to convert it in to metallurgical alumina, is often directly used as alumina trihydrate as raw material for uses other than aluminum manufacture. For these uses, the presence of anhydrous alumina in the trihydrate cannot be accepted.
Depending on the calcination degree, the sodium oxide (Na.sub.2 O) content of the starting hydrate and its graduation, different types of alumina are obtained with a higher or lower content in alpha phase (.alpha..Al.sub.2 O.sub.3), as well as in other transition phases such as gamma, theta, etc. phases, and, in a lower amount in dependence of the sodium oxide being present, in beta phase (11.Al.sub.2 O.sub.3.1Na.sub.2 O).
Further to its use as raw material for the production of aluminum and thanks to its interesting physical properties (high melting temperature, good dielectric characteristics, particularly high hardness, chemical inertia, etc.), alumina is nowadays an essential product in a great deal of industrial fields and for a wide range of uses. We may quote the manufacture of high temperature refractory products, industrial ceramic products and porcelains, enamels and special glass, abrasives, electrofluxes, etc.
The different types of special aluminas are mainly distinguished by their different calcination degree and, therefore, by their content in alpha alumina (.alpha..Al.sub.2 O.sub.3), by their content in sodium oxide, by their purity in general and by their granulometric distribution and monocrystal size.
The calcined alumina, when coming out of furnaces, is formed by monocrystal aggregates, the diameter of which can vary depending on the granulometric characteristics of the starting hydrate. This alumina can be used such as it comes out of the furnace, or by separating the agglomerates into their elemental components by a grinding process. In this case, an extremely thin product is obtained, constituted by elemental crystals or monocrystals, so that almost all particles have a size below 44 microns.
The added value of the special alumina generally increases the more calcined it is (high content in alpha phase), the less sodium it has and the thinner it is.
Criteria for evaluation of the quality of an alumina as raw material for ceramic products are cited as for instance hereinafter:
Chemical purity: In this case the content in sodium oxide (Na.sub.2 O) plays a decisive role. It has been demonstrated that the mechanical and dielectric properties of ceramic oxides are improved as the beta alumina rate decreases. For high quality products, the content in sodium oxide (Na.sub.2 O) must not exceed the 0.1%; for less strict uses, the acceptable level stands between 0.2 and 0.3%.
Primary crystal size: Alumina individual particles consist of small crystal aggregates, "primary crystals". They generally size from 1 to 10 microns. These "primary crystals" must have a small size in order to assure a high sintering activity and, consequently, a high final density at relatively low treatment temperatures. A greater size of the primary crystals makes the compacting density increase and, therefore, the contraction decreases during the thermal treatment. By selecting the adequate alumina type, the behaviour during the thermal treatment can be modified depending on the type of manufacturer's technical installations.
Content in alpha (.alpha..Al.sub.2 O.sub.3): The temperature, the residence time and the additive amount used in calcination determine the rate of the produced alpha variant (corindon). It is desirable to achieve more than a 90% (more than a 95% if possible) in order to minimize contraction during the thermal treatment.
Graduation: So as to obtain a complete sintering of the ceramic material even at temperatures below 1800.degree. C., it is required that the alumina has the required size for the primary crystal. This usually achieved by a grinding process.